Manufacturers of primary alkaline batteries have long recognized the existence of internal gassing that occurs prior to discharging a cell. This phenomenon, commonly referred to as "shelf gassing," may pressurize a cell such that the flat bottom of a cylindrical battery becomes bulged. One problem created by severely bulged cell bottoms is that the cell may be too long to fit into a battery compartment that is located within a battery operated device. In severe cases, the pressure buildup may activate the cell's safety vent. Electrolyte expelled through the vent can injure people and/or damage the device in which the cell has been placed.
Several methods have been used to minimize the shelf gassing problem. First, the best known method relies upon amalgamation of the anode's zinc surface. This action raises the hydrogen evolution overvoltage on the zinc's surface. As the overvoltage increases, the rate of hydrogen gas generation is restricted. Second, a variety of organic compounds have been used to coat the surface of the zinc. Representative examples of organic corrosion inhibitors can be found in U.S. Pat. No. 3,847,669, or U.S. Pat. No. 4,195,120, which is directed toward the use of organic phosphate esters, or U.S. Pat. No. 4,606,984, which describes a fluorinated molecule that contains an ethoxylated polymer unit. Third, the anode's zinc particles have been altered by incorporating minor quantities of elements such as indium, lead, thallium and/or gallium into the zinc. This combination of zinc and at least one other element may take the form of a surface deposit or an alloy. Fourth, the zinc particles have been modified by heating in order to reduce the number of grain boundaries. Since the gas generating reactions are thought to occur primarily at the grain boundaries, the quantity of gas generated may be reduced as the number of grain boundaries per particle of zinc is reduced. While all of these gassing inhibitors have been claimed to be effective at reducing cell gassing, none of the inhibitors have effectively demonstrated the ability to limit gassing to a very low level in a mercury free cell or in a cell that contains very little mercury.
In alkaline batteries employing conventional type brass anode current collectors, internal gassing is still a major problem. Conventional type brass employed as an anode current collector has a composition of about 65% to 70% copper and about 30% to 35% zinc. With the elimination of mercury in the battery, internal gassing is even more of a problem since it can cause the housing of the battery to expand, i.e., bulged cell bottom, even when using corrosion inhibitors.
It is an object of the present invention to provide an alkaline electrochemical cell with a zinc anode having little or effectively no internal gassing during storage.
It is another object of the present invention to provide a mercury free alkaline electrochemical cell with a zinc anode having little or effectively no internal gassing during storage.
It is another object of the present invention to provide a mercury free alkaline electrochemical cell with a zinc anode and a conductive anode current collector having on its surface a layer of a discontinuous dispersion of lead and wherein said cell has little or effectively no internal gassing during storage.
It is another object of the present invention to provide a mercury free alkaline electrochemical cell with a zinc anode and a conductive anode current collector comprising a substrate having on its surface a layer of a discontinuous dispersion of lead in a lead and zinc mixture and wherein said cell has little or effectively no internal gassing during storage.
It is another object of the present invention to provide a mercury free alkaline electrochemical cell with a zinc anode and an anode current collector comprising a copper-zinc alloy substrate coated with a lead-containing mixture and wherein said cell has little or effectively no internal gassing during storage.